Texture Painting - wrap brush to mesh?
You can use the Project Paint settings to get around this - by default, Occlude, Cull and Normal are all on, as well as a 2 pixel bleed. You can uncheck these like I show in the picture, and then your paint will paint through the mesh all the way like a blade - and if you want to only do some of it, you can use the Face Selection masking in the header window, right next to the drawtype icon.